In all regions throughout the world the use of frequency bands for radio transmission are regulated. The regulations consider many aspects such as transmitted output power, spurious emissions, adjacent carrier leakage etc and these requirements vary throughout the different regions and for different frequency bands. One main differentiator for the frequency band regulations is if they are classified as licensed bands or unlicensed bands.
For a licensed frequency band, the band will be separated into one or many carriers and each of these carriers will be associated with one single user. In case of telecommunication this will typically be a Telecom operator. The operator then has full access to the carrier and can transmit continuously on that carrier. Sometimes the license to transmit is also limited to a certain type of wave-form transmission such as e.g. GSM, WCDM or LTE.
For an unlicensed frequency band, on the other hand, anyone can use the different carriers within that band as long as the transmitted waveform is following the regulator limitations. These limitations can describe the output power, the channel occupancy time, the access procedure etc. Examples of such frequency bands are for example the 2.4 Ghz or 5.1 GHz commonly used by different WiFi systems. Normally the transmission on these frequency bands is shared between many technologies and the transmissions are required to be limited in time (non-continuous) to allow others to use the available spectrum.
Comparing WiFi and LTE it is notable the WiFi is well-suited for use on unlicensed carriers whereas LTE is better suited for use on licensed carriers. One main differentiator is that WiFi is an asynchronous access technology since WiFi transmissions can happen at any time if the channel is free. Moreover, the WiFi transmissions have a variable size which is signalled in the preamble of the WiFi data frame. LTE on the other hand is a synchronous access technology where transmissions follow a rigid frame structure and are required to be aligned to the synchronization signals broadcasted in each radio frame. Each scheduled data transmission also has a fixed length of 1 ms which is equal to one LTE subframe.
License-Assisted Access-Long Term Evolution (LAA-LTE) is one of the main work items for the 3GPP LTE Release13 standard. It proposes to use unlicensed bands (e.g. 2.4 GHz and 5.1 GHz) for LTE or LTE-like transmission in coexistence with other wireless standards (like WLAN IEEE 802.11 and Bluetooth). The idea is to have a primary channel on LTE in a licensed band to serve as the main connection while secondary carrier(s) are set up in unlicensed bands to boost the throughput to the user.
Since LAA-LTE has the benefit of always having the possibility to transmit data on the licensed carrier it is considered to be a big advantage to use the licensed carrier for all kinds of control signalling such as grants, acknowledgements/negative-acknowledgments, etc. The unlicensed carrier can be used to off-load the licenses carrier for data whenever it is available. In this way the licensed carrier can be used for robust control signalling and the unlicensed carrier mainly used to boost user data rates in a best-effort fashion.
The main issue with such a solution is the coexistence between many different access technologies since collisions in transmission may significantly reduce performance for all involved access methods and thereby lead to poor spectral efficiency. One of the main goals of the 3GPP work item is to find a solution to handle coexistence with other access technologies such as WiFi and also coexistence between several LTE networks being run by different operators. This leads to the following two main differences of LAA-LTE compared with regular LTE on a licensed carrier:
Since it is not allowed to transmit continuously on the unlicensed carrier and since LTE is synchronous by definition the transmissions on the unlicensed carrier need to be synchronized and aligned to the frame structure on the licensed carrier. This makes it possible for terminals to detect and decode signals on the unlicensed carrier even though transmissions are non-continuous on the unlicensed carrier.
In many cases it is required to listen to the unlicensed carrier first to determine it is available to avoid interrupting ongoing transmissions. This is solved by some carrier sensing functionality that detects the presence of other transmitters on the carrier before starting to transmit. If the carrier is occupied the transmission has to be postponed until the carrier is available.